NOx-concentration detecting apparatus for detecting the concentration of NOx (NOx) contained in exhaust gases from internal combustion engines and the like using an NOx sensor are disclosed, for example, in European Patent Application Laid-Open No. 0678740A1 and SAE Paper No. 960334, pp. 137-142, 1996. An NOx sensor used in such a conventional NOx-concentration detecting apparatus is composed of oxygen-ion conductive solid electrolyte layers that define a first measurement space and a second measurement space. The first measurement space communicates with a gas to be measured (hereinafter called "a measurement gas") via a first diffusion-controlling layer, and the second measurement space communicates with the first measurement space via a second diffusion-controlling layer. Furthermore, the solid electrolyte layer of the first measurement space is sandwiched between porous electrodes so as to form a first oxygen-pumping cell and an oxygen-concentration-measuring cell. Also, the solid electrolyte layer of the second measurement space is sandwiched between porous electrodes so as to form a second oxygen-pumping cell.
In the thus-configured NOx-concentration detecting apparatus, current is applied to the first oxygen-pumping cell such that the output voltage from the oxygen-concentration-measuring cell achieves a predetermined value, thereby controlling the concentration of oxygen contained in the first measurement space at a constant level. At the same time, a constant voltage is applied to the second oxygen-pumping cell to thereby pump out oxygen from the second measurement space. At this time, the NOx concentration of a measurement gas can be obtained by measuring the current flowing through the second oxygen-pumping cell.
A measurement gas, e.g., exhaust from an internal combustion engine or the like, contains gas components other than NOx, such as oxygen, carbon monoxide and carbon dioxide. Thus, in the aforementioned NOx-concentration detecting apparatus, first, the first oxygen-pumping cell is activated so as to control the concentration of oxygen contained in the first measurement space to a very low level. Then, in the second measurement space into which the measurement gas controlled to a low oxygen concentration flows, a constant voltage is applied to the second oxygen-pumping cell in a direction such that oxygen is pumped out from the second measurement space. As a result, NOx contained in the measurement gas is decomposed into nitrogen and oxygen by means of the catalyzing function of the porous electrodes of the second oxygen-pumping cell, and the thus-generated oxygen is then pumped out from the second measurement space. Thus, the NOx concentration of the measurement gas can be obtained by measuring the current flowing through the second oxygen-pumping cell with no influence of other gas components contained in the measurement gas.
In an NOx-concentration measuring apparatus of this kind, in order to accurately detect the concentration of NOx by the detecting method described above, the sensor must be heated to a predetermined active temperature (for example, 800.degree. C. or higher) so as to activate the cells. Therefore, a heater for heating the sensor is additionally provided.
In recent years, in order to improve fuel consumption and attain high efficiency for internal combustion engines using gasoline as a fuel, internal combustion engines have been developed that are controlled so as to operate at a lean air-fuel ratio, where the amount of air is large with respect to that of the fuel (lean burn engines, direct injection engines, etc.). Normally, in an internal combustion engine, NOx and unburned components (HC and CO) contained in exhaust gas are made to react with each other using a three-way catalytic converter to reduce NOx to N.sub.2, thereby purifying the exhaust gas. In operating at a lean air-fuel ratio, a large amount of oxygen is contained in the exhaust gas. As a result, the oxygen reacts with unburned components, resulting in a failure to remove NOx.
To solve the above problem, a so-called NOx occlusion catalyst is used, which is a three-way catalytic converter containing an NOx storage material for storing NOx contained in exhaust gas in the form of nitrate. However, because of the limited amount of NOx that can be occluded in the NOx occlusion catalyst, the control for refreshing the NOx occlusion catalyst so as to restore its capacity for storing NOx is performed in the following manner. The air-fuel ratio of a mixture fed to an internal combustion engine is temporarily controlled to a rich air-fuel ratio, where the amount of fuel is relatively large, before the amount of stored NOx reaches the above capacity limit. The resulting exhaust gas contains a large amount of unburned components from the internal combustion engine. This causes the unburned components to react with NOx stored in the NOx occlusion catalyst, thereby refreshing the NOx occlusion catalyst so that its capability of storing NOx is restored.
Such a refreshing operation is performed periodically at constant intervals or when the amount of leakage of NOx from the NOx occlusion catalyst exceeds a predetermined level. In the latter case, the amount of NOx leakage is detected using the NOx-concentration measuring apparatus described above, including the NOx sensor installed in an exhaust passage of the internal combustion engine at a location downstream of the NOx occlusion catalyst.
In order to perform accurate control, the NOx sensor used in such an application must at least be able to determine the NOx concentration in units of 100 ppm.